1. Field of the Invention
The present invention relates to a swash plate type refrigerant compressor using CO2 as a refrigerant. More particularly, the present invention relates to a swash plate type piston-operated refrigerant compressor incorporating therein pistons reciprocating to compress the refrigerant and having an improved sliding performance and an extended operating life.
2. Description of the Related Art
Generally, a single-headed piston operated swash plate type compressor used for a vehicle climate control system includes a swash plate or a cam plate mounted on the drive shaft in a crank chamber, so that the rotation of the swash plate cooperating with the drive shaft is converted into the linear motion of the pistons inserted in cylinder bores. With the reciprocation of the pistons, the refrigerant gas returning from an external refrigeration system is sucked into the cylinder bores from a suction chamber and, after being compressed, is discharged into a discharge chamber. Specifically, many single-headed swash plate type compressors are so configured that the refrigerant returned gas is introduced directly into the cylinder bores without passing through the crank chamber as described above. The lubrication of the sliding portions and elements arranged in the crank chamber, therefore, are primarily dependent on the lubricant supplied to the crank chamber together with the blow-by gas.
The amount of the blow-by gas depends on the size of the fitting gap between the cylinder bores and the pistons. For supplying enough lubricant to properly lubricate the sliding portions and elements in the crank chamber, the fitting gap is required to have an appreciable size. In such a case, the problem of reduced compression efficiency is posed.
The practical application of CO2 as a replacement refrigerant has recently been favored for environmental protection. Nevertheless, with a compressor using CO2 (carbon dioxide gas) as a refrigerant, it is difficult to satisfy the pressure requirements. In a compressor employing an ordinary simple seal method with the cylinder bores and the pistons snugly fitted with each other without using any special sealing means between them, the amount of blow-by gas extremely increases to deteriorate the compressing performance. In view of this, a piston ring, which has thus far attracted little attention for application to an air-conditioning compressor, has recently become important.
Even when the piston ring is used, however, the large difference of the pressure acting on the operating end and the rear end of each piston at the time of compression and the high density of the refrigerant gas increases the gas flow rate, in the same passage area, considerably over the conventional compressor using the fluorinated hydrocarbon gas.
When the pistons move from the bottom dead center toward the top dead center for compressing the refrigerant gas, the compression reaction force and the inertia force of the pistons act on the swash plate, and the force thus acting on the swash plate is exerted on the pistons as a reaction force. In view of the fact that the swash plate is inclined with respect to a plane perpendicular to the center axis of the drive shaft, part of the force acting on the pistons is exerted in such a direction as to press the pistons against the inner periphery of the cylinder bores. Namely, the respective pistons receive side forces from the inner peripheral surface of the corresponding cylinder bores. Especially in the case of the CO2 refrigerant, the side force is so great that the pistons unavoidably come into direct contact with the cylinder bores even if piston rings are fitted on the pistons.
Accordingly, an object of the present invention is to provide a swash plate type piston-operated refrigerant compressor using the CO2 refrigerant in which the blow-by gas amount is limited in cooperation with the piston ring mounted on the pistons while at the same time preventing direct contact between the cylinder bores and the pistons made of metals of the same type.
Another object of the invention is to provide a swash plate type refrigerant compressor in which superior lubrication of the piston sliding portion is secured and a sufficient amount of lubricant can be supplied to the sliding elements and portions including the swash plate, the shoes, the hinge mechanism and the bearings in the crank chamber.
In accordance with the present invention, there is provided a swash plate type refrigerant compressor which comprises:
at least a casing having at least a cylinder bore and a crank chamber;
a drive shaft supported rotatably on the casing;
a swash plate mounted around the drive shaft to be rotated simultaneously with the drive shaft in the crank chamber; and
at least a piston having a top portion inserted into the cylinder bore for compression operation;
wherein the piston operatively engaged with the swash plate acts in the cylinder bore to compress the CO2 refrigerant in response to the rotation of the drive shaft;
wherein a peripheral wall extending around the cylinder bore and the piston is formed of an aluminum alloy as a base metal; and
wherein the piston has a central axis and an outer peripheral surface, formed around the central axis, coated with a film of fluororesin material, the piston being provided with a piston ring mounted at a position adjacent to the top portion of the piston.
In the described compressor, the blow-by gas amount is determined by the width of the closed gap of the piston ring and the fitting gap between the cylinder bores and the pistons. Since the fluororesin film is formed on the outer peripheral surface of the pistons, however, direct contact is surely avoided between the metals, of the same type, of the cylinder bores and the pistons. Thus, the fitting gap is minimized so that the blow-by gas amount, i.e. the leakage amount of the compressed refrigerant is reduced to prevent the reduced performance of the compressor. At the same time, the surface contact through the fluororesin film can sufficiently resist a large side force.
Preferably, the casing having the cylinder bores is formed of a hypereutectic aluminum-silicon alloy and the piston ring is made of an iron metal.
The use of a hyper eutectic aluminum-silicon alloy for the casing as described above makes it possible to sufficiently resist the sliding with the piston ring made of an iron metal.
Also, preferably, in a compressor having a first oil groove extending in the peripheral direction in parallel and below a piston ring groove in which the piston ring is mounted, and a second oil groove extending along an axial direction below the first oil groove, the lubricant passage area can be increased for a lower viscous resistance without increasing the gas flow rate. Therefore, the lubricant can be held in the fitting boundary with the cylinder bores.
Further, assume that the second oil groove is formed in such a position as to be partly exposed to the interior of the crank chamber at least when the pistons reach the bottom dead center. Even when the refrigerant compressor is of variable displacement type with an extremely small angle of inclination of the swash plate, the lubricant is positively supplied into the crank chamber from the second oil groove, and therefore superior lubrication is achieved. Furthermore, in the case where the second oil groove is formed on the outer peripheral surface of the pistons where the effect of the side force can be avoided as far as possible, the second oil groove is not strongly pressed against the cylinder bores. Therefore, the wear and damage to both the pistons and the cylinder bores can be prevented.